1. Field of the Invention
The present invention relates an optical transmission system with Raman amplifiers comprising a supervisory system. The present invention also relates to an optical repeater.
2. Description of the Related Art
In an optical transmission system, especially in a submarine optical transmission system, there is the very felt need of supervising the operating state of devices disposed along the optical line, such as for example optical repeaters, branching units and so on. In the particular case of a submarine system, such supervising has to be performed remotely.
In the present description and claims, the expression “supervisory signal” is used to indicate either command informations suitable to set predetermined system parameters (such as, for example, the gain and the output power of an optical amplifier), or query informations suitable to check the operating state of a device, or informations on the operating state of said device, or communications between the maintenance and/or supervisory personnel operating in intermediate points of the optical line or in terminal stations of the system.
For example, the supervising of the operating state of an optical repeater typically comprises control functions, such as for example the regulation of the value of the current furnished to the pump lasers, or the switching between a working pump laser and a spare pump laser, and so on. Further, the supervising of the operating state of an optical repeater typically also comprises monitor functions, such as for example the monitoring of the operating temperature of the repeater, or the monitoring of the current furnished to the pump lasers, or the monitoring of the input power of the optical signal entering into the repeater and/or the monitoring of the output power of the optical signal exiting from the repeater, and so on.
For the purpose of supervising, many kinds of supervisory systems in optical transmission systems have been proposed.
For example, EP0504777 discloses a transmission system for transmitting over an optical link an auxiliary signal that is auxiliary relative to a traffic signal which is transmitted simultaneously over said link, the system including at least one equipment including optical amplification means. For the purpose of enabling said equipment to emit the auxiliary signal, the system includes means for modulating the gain of the optical amplification means of said equipment by said auxiliary signal. Optical amplification means disclosed in '777 patent are rare-earth doped fiber amplifiers and semiconductor amplifiers.
EP0675610 teaches to modulate the pump radiation of an erbium doped fiber amplifier through a modulating signal carrying supervisory informations. Such modulating signal has a high modulation frequency, that is, a modulation period that is less than the fluorescence time of erbium ions, so as not to affect the gain of the erbium doped fiber amplifier. In this way, the supervisory informations are sent using as optical carrier the excess pump radiation that does not contribute to the erbium doped fiber amplifier pumping.
U.S. Pat. No. 5,625,481 teaches to modulate the spontaneous emission of an erbium-doped optical fiber amplifier with a supervisory signal through a band pass optical filter whose transmission characteristic is changed in function of the supervisory signal.
U.S. Pat. No. 6,111,687 teaches to use a band pass optical filter for modulating an optical signal in output from an optical amplifier with such amplitude and frequency as to not disturb the data transmission performed by the optical signal. Such modulation allows the optical amplifier to transmit supervisory messages.
EP0751635 describes a supervisory system for a WDM optical communication system for transmitting a command signal from a terminal station to an erbium-doped optical fibre amplifier and response signals from an erbium-doped optical fibre amplifier to the terminal station. A first method described for transmitting the command signal consists in using the same command signal to directly modulate, one by one, a plurality of optical sources that generate laser beams at different wavelengths. The laser beams at different wavelengths are then externally modulated by the respective main signals to be transmitted along the system and thus, wavelength multiplexed. According to a second method, the laser beams at different wavelengths are first externally modulated by the respective main signals, then they are wavelength multiplexed in a single WDM optical signal; afterwards, the latter is externally modulated in function of the command signal through a lithium niobate modulator (LiNbO3). On the other hand, as regards the response signals sent by the erbium-doped optical amplifiers to the terminal stations, they are transmitted by directly modulating the pump source of the optical amplifiers in function of the response signal to be transmitted so as to modulate the gain of the erbium-doped optical amplifiers. The command signals have a frequency in the range of 10 MHz whereas response signals have a frequency in the range of KHz.
Fiber Raman amplifiers have been attracting a great attention, because of their capability to increase the transmission capacity and/or repeaterless span lengths. Raman amplification is an amplification by which energy is transferred from an electromagnetic pump wave to a lower frequency signal wave via a molecular vibration. The responsible mechanism is stimulated Raman scattering (SRS).
Raman amplifiers offer several advantages, such as a low noise, a greater flexibility in choosing the signal wavelength and a broad gain bandwidth. The greater flexibility in choosing the signal wavelength mainly depends on the fact that the Raman peak of a material, exploited for the amplification of the signal, is dependent practically only on the pump wavelength, differently from what happens for example in erbium-doped fiber amplifiers, in which the choice of the signal wavelength is restricted by the stimulated emission cross-section of the erbium. The broad gain bandwidth of Raman amplifiers can be much enlarged, for example by using multiple pump sources. Such a broad gain bandwidth may represent the possibility to extend the usable optical bandwidth outside the conventional C-band and the extended L-band of the erbium-doped fiber amplifiers.
The Applicant has faced the problem of implementing a supervisory system in an optical system comprising Raman amplifiers.
It is known that the gain of a Raman amplifier may be modulated according to a supervisory signal to be sent on optical link.
For example, U.S. Pat. No. 6,188,508 discloses a control signal superimposer for superimposing a control signal on a signal light, comprising: a pumping light source for generating a pumping light with intensity fluctuation in accordance with the control signal; a Raman amplification medium pumped by the pumping light from the pumping light source for Raman-amplifying the signal light; a combiner for combining the pumping light output from the pumping light source and the signal light to be Raman-amplified and then supplying them to the Raman amplification medium; and an optical filter for extracting the signal light component from the output light of the Raman amplification medium and terminating the pumping light component. In the '508 patent it is disclosed that in order to superimpose the control signal on the 1.5 μm band signal light through the Raman amplification, it is necessary to propagate the 1.5 μm band signal light desired to be amplified and the pumping light in the same direction.
A control signal superimposer of the kind disclosed in '508 patent is disclosed also in the article of H. Maeda et al., “Remote Supervisory System of FSA-WDM System”, NTT Review, vol. 12, n.4 (2000), pages 25-31. A control command corresponding to a repeater supervision item is issued. The terminal equipment converts the received command to a supervisory signal of low-speed subcarrier and superimposes the supervisory signal on the WDM signal for transmission along a transmission line with repeaters. A repeater sends the response signal to the terminal equipment at both ends of the transmission line, or performs repeater state control. Raman amplification is exploited for supervisory signal transmission from the terminal equipment. The configuration of the supervisory signal modulator uses dispersion shifted fiber (DSF) for Raman amplification. A subcarrier-ASK signal is used to intensity-modulate the 1.48 μm pumping source. The resulting signal co-propagates along the DSF with the WDM signal and Raman-amplifies the WDM signal according to the subcarrier-ASK signal. This WDM signal is then sent to the submarine transmission line. A repeater response signal from the submarine optical repeater is superimposed on the amplified spontaneous emission (ASE) light produced by the repeaters when the system has failed or is out-of-service.
EP1122898 discloses an optical repeater monitoring system comprising an oscillating source, a reference signal transmitter for transmitting a reference signal of a predetermined frequency generated by an output of the oscillating source to a first optical fiber, and an optical repeater. The optical repeater has a first photodetector for converting light from the first optical fiber into an electrical signal, a reference signal extractor for extracting a component of the reference signal from an output of the first photodetector, a carrier generator for generating a carrier from an output of the reference signal extractor, a monitor signal modulator for modulating the carrier generated by the carrier generator with a monitor signal showing an operating state of the optical repeater, a transmitter for transmitting an output of the monitor signal modulator to a second optical fiber. Optical amplifiers (e.g. optical amplifiers using erbium-doped optical fiber), comprised in the optical repeater, are pumped by a pumping circuit and then optically amplify signal lights from the first and the second optical fiber. The output of the monitor signal modulator is applied to the pumping circuit. The pumping circuit weakly modulates the intensity of the pumping light to be transmitted to the optical amplifiers according to the output from the monitor signal modulator. The gain of the optical amplifiers is thus modulated by the output of the monitor signal modulator in order to transmit the monitor signal to a terminal station. In the '898 patent application, it is disclosed that such technique is also applicable to Raman amplification: in such case, pumping light for leading the Raman amplification within the wavelength band of the signal light is applied to the optical fiber line and the intensity of the pumping light is modulated with the output of the monitor signal modulator. As a result, the gain of the signal light propagating on the optical fiber line fluctuates according to the output of the monitor signal modulator and thus brings the same effect with the case in which the gain of the optical amplifiers is fluctuated.
However, the Applicant has found that a signal light modulated with a low frequency subcarrier entering in a Raman amplifier behaves in different manners if the pump radiation co-propagates with the signal light (that is, propagates in the same direction of the signal light) or counter-propagates with respect to the signal light (that is, propagates in the opposite direction with respect to the signal light). In fact, if the signal light and the pump radiation counter-propagate in the Raman amplifying medium, the low frequency subcarrier superimposed to the signal light is substantially left unchanged if the low frequency subcarrier lays in a band of frequencies higher than some kHz, that is, in a typical band of frequencies for a supervisory signal. On the other hand, if the signal light and the pump radiation co-propagate in the Raman amplifying medium, the low frequency subcarrier superimposed to the signal light may be attenuated in a substantial manner. This may cause the loss of the supervisory signal at the end of a chain of co-propagating Raman amplifiers. Herein and in the following, by “co-propagating Raman amplifier” it is to be intended an amplifier in which the pump radiation and the light to be amplified propagate in the Raman medium (e.g. an optical fiber) in the same direction; by “counter-propagating Raman amplifier” it is to be intended an amplifier in which the pump radiation and the light to be amplified propagate in the Raman medium in opposite directions.
As disclosed in the U.S. Pat. No. 6,188,508 cited above, a low frequency modulation of the pump source providing the pumping radiation of a Raman amplifier superimposes such frequency modulation to a signal light amplified in the Raman amplifier if the pump radiation and the signal light co-propagate in the Raman medium. On the other hand, the Applicant has verified that if the pump radiation and the signal light counter-propagate in the Raman medium, no superposition of the low frequency modulation on the signal light practically occurs at frequencies of the order of one kHz and above. In such conditions, a supervisory signal carrying a sufficient quantity of informations could hardly be superimposed to a signal light by modulation of the gain of a counter-propagating Raman amplifier.
Summarizing what said above, the Applicant has found that in an optical transmission system comprising a chain of Raman amplifiers, the following problems may arise, for the implementation of a supervisory system in the optical transmission system:                a) if the Raman amplifiers are co-propagating, each of them could be gain-modulated in order to superimpose a supervisory signal to a signal light propagating through the chain. However, each Raman amplifier would substantially attenuate a supervisory signal superimposed on the signal light by a previous Raman amplifier of the chain or Raman amplifiers or by a terminal station, so that such supervisory signal may be lost at the end of the chain of Raman amplifiers;        b) if the Raman amplifiers are counter-propagating, a supervisory signal superimposed to a propagating signal light may pass through the chain of Raman amplifiers with substantially no attenuation. However, the counter-propagating Raman amplifiers may hardly be gain-modulated in order to provide such supervisory signals by superposition on the signal light.        